Thermoplastic polyurethanes are normally made by reacting three essential components. The first component is a polyol, which is a hydroxyl terminated intermediate. The second component is a multifunctional isocyanate, preferably a diisocyanate. The third component is a glycol chain extender, usually a short chain glycol. The three components react to form the TPU polymer. TPUs are classified by types, such as polyether, polyester, and polycarbonate depending on the type of polyol used to make the TPU. For example, a polyether TPU would use a polyether polyol and a polyester TPU would use a polyester polyol, and so on.
It is well known that polyether based thermoplastic polyurethane (TPU) has very good hydrolysis resistance, that is when exposed to water. Unfortunately, polyether TPU has relatively poor thermal resistance. Polyester based TPU has poor hydrolysis resistance but has good thermal resistance and high physical properties, such as tensile strength. The proper selection of a particular TPU for a given end-use application will depend on the environment the TPU will be exposed to in the product use.
It is also known that many physical properties of a TPU can be improved by lightly crosslinking the TPU polymer, usually by adding a multifunctional isocyanate, preferably a diisocyanate. The isocyanate is typically added in a compounding step where the TPU polymer is added to a melt mixing machine, such as a twin screw extruder and the isocyanate is added to the extruder. The compounded polymer is pelletized for further processing to make end products, such as extruded profiles, sheets, and the like, or injection molded into various articles. The addition of the isocyanate to the TPU causes the TPU to crosslink and thus the mixing torque increases and it becomes difficult to process.
It would be desirable to have a TPU which has the good properties of both the polyether TPU and the polyester TPU. It would also be desirable to have a crosslinked TPU which can be processed on melt mixing equipment.